(1). Field of the Invention
The present invention relates to a system for exhaust gas purification, employed for the purification of harmful substances such as hydrocarbons (HC), nitrogen oxides (NO.sub.x), carbon monoxide (CO) and the like, present in the exhaust gases discharged from internal combustion engines of automobiles, etc., as well as to a method for exhaust gas purification using the above system.
(2). Description of the Prior Art
Various catalyst techniques have heretofore been proposed for the purification of the exhaust gases discharged from internal combustion engines of automobiles, etc.
For example, in WO 93/07363 is disclosed an apparatus for exhaust gas purification, which is an NO.sub.x absorbent disposed in the flow path of exhaust gas so that the absorbent can absorb NO.sub.x when the air fuel ratio of exhaust gas is high (fuel is lean) and can release NO.sub.x when the oxygen concentration exhaust gas is low.
Also in U.S. Pat. No. 4,902,487 is disclosed a technique of collecting the particulates contained in diesel engine exhaust gas by the use of a filter and burning the particulates at low temperatures of about 225.degree.-300.degree. C. by using NO.sub.x as an oxidizing agent.
Further in Catalysis Today, 22 (1994), 113-126, it is disclosed that when a so-called "Pd only catalyst" is used with a basis element and/or a transition metal oxide added thereto, catalyst poisoning by HC is reduced by the electron-donating effect of the added element and/or oxide and, as a result, an improved NO.sub.x purification ability is obtained.
In recent years, it has been one of the major technical tasks to effectively purify the combustible components (e.g. HC, CO and H.sub.2), particularly HC present in the exhaust gas discharged from gasoline engine during its cold start. During the cold start, engines are operated generally at a low air fuel ratio (fuel is rich) and a large amount of HC is contained in the exhaust gas; meanwhile, the catalyst provided in the exhaust gas system does not yet reach the activation temperature and shows a low purification ability; consequently, a large proportion of the HC tends to be released into the atmosphere without being purified. Therefore, there is needed a technique capable of effectively purifying the HC present in the exhaust gas discharged during cold start.
When the above-mentioned conventional techniques are looked at from the above standpoint, the apparatus disclosed in WO 93/07363 aims at the purification of the NO.sub.x discharged from lean burn engine and diesel engine during steady-state operation and does not aim at the purification of the HC discharged during cold start; further, the apparatus includes no idea of effectively utilizing the reaction heat generated. Furthermore, in this apparatus, the air fuel ratio of exhaust gas is intentionally switched from a high side (fuel is lean) to a stoichiometric air fuel ratio or a low side (fuel is rich) before the NO.sub.x absorbent reaches its saturation, whereby the NO.sub.x absorbed by the absorbent is released and reduction and purification are allowed to take place. The switch of air fuel ratio from high side(lean fuel) to stoichiometric ratio or low side (rich fuel) is repeated and the apparatus is used at high temperatures for a long period of time.
The technique disclosed in U.S. Pat. No. 4,902,487 is for the treatment of the particulates contained in diesel engine exhaust gas and, as is so with the apparatus mentioned above, does not aim at the purification of the exhaust gas discharged from gasoline engine during cold start. Further in this technique, while the NO.sub.2 used for burning particulates is generated from the NO contained in exhaust gas, by the catalysis of the Pt used, no idea of using occluded or concentrated NO.sub.2 is employed and therefore it is difficult to supply NO.sub.2 of an amount sufficient to treat the diesel particulates. Furthermore, since the particulates contained in diesel engine exhaust gas are solid particulates having an average particle diameter of about 0.2 .mu.m, the particulates, unlike the combustible components (e.g. HC) contained in gasoline engine exhaust gas, only accumulate on a precious metal (a catalyst) and are not adsorbed thereby; consequently, the probability of diesel particulates to react with NO.sub.2 on the precious metal is considered to be low.
With respect to the technique disclosed in Catalysis Today, 22 (1994), 113-126, even if the electron-donating effect of the added element and/or oxide can enhance the NO.sub.x purification ability of the "Pd only catalyst", i.e. the NO.sub.x light-off performance, said effect alone is unable to effectively purify the HC generated in a large amount during cold start.
In order to increase the purification ability of HC generated during cold start, there was proposed a system for exhaust gas purification disposed in an exhaust gas pipe, comprising (a) a catalyst and (b) an adsorbent (e.g. zeolite) having HC adsorbability, provided upstream of the catalyst. This system aims at temporary adsorption of HC on the adsorbent until the downstream side catalyst reaches its light-off temperature; however, in actuality, the HC adsorbed on the adsorbent begins to be desorbed from the adsorbent when the adsorbent reaches about 150.degree. C., and the downstream side catalyst does not light-off until it reaches about 300.degree.-350.degree. C. Thus, even when the adsorbent is provided upstream of the catalyst, as long as the catalyst is a conventional catalyst having a high light-off temperature, the difference between the initial temperature of HC desorption from adsorbent and the light-off temperature of catalyst is too large, making impossible the effective purification of HC.